The objectives of the proposed research are: (1) to measure the mechanical properties of cardiac muscle; and (2) to shed light on the molecular mechanism underlying contraction in cardiac muscle. To these ends we will employ primarily thin preparations of trabecular specimens from the right ventricles of rats, and where appropriate, frog atrial trabecular strands. These preparations will be mounted in an apparatus which will be used to control and/or measure muscle length, tension, or sarcomere length. The latter is measured on-line using an optical diffraction technique with high resolution sensors we have developed. The projected series of experiments will include measurements of the length-tension relations, force-velocity relations, and elastic properties of cardiac sarcomeres. These experiments will be interpreted in terms of presently accepted and newer views on the molecular mechanism of contraction.